How Astrian calculates your chart

The ephemeris: NASA JPL DE441

Astrian is built on a purpose-designed astronomical engine that reads directly from NASA's JPL Development Ephemeris 441 (DE441) — the same mathematical model used for interplanetary navigation. DE441 covers 13,000 years of planetary positions with sub-arcsecond precision (better than 1/3600 of a degree).

Most astrology software today relies on ephemeris libraries derived from older JPL models. The differences are small in absolute terms, but they become meaningful near sign boundaries, where a fraction of a degree determines which sign a planet falls in.

The engine behind Astrian

The engine behind Astrian was built from scratch as a dedicated astronomical computation system. It is not a wrapper around existing libraries — it implements the JPL planetary theory directly, reading binary ephemeris files and computing geocentric positions through the full heliocentric-to-geocentric transformation pipeline.

The engine takes a birth date, time, and geographic coordinates, queries DE441, and returns planetary positions in the tropical zodiac along with house cusps, aspects, and derived points. It runs over 1,600 automated tests on every update, covering 41 calculation endpoints and 6 reference charts spanning dates from 1900 to 2100.

What we calculate

For each chart, the engine computes the ecliptic longitude of the Sun, Moon, and planets (Mercury through Pluto), plus Chiron and the lunar nodes. Positions are geocentric — as seen from Earth — in the tropical zodiac (0° Aries = the vernal equinox).

House cusps are computed in Placidus (default), Koch, Whole Sign, Porphyry, and Equal House systems. We publish a house system comparator so you can see how different systems affect your chart's structure without recalculating from scratch.

Aspects are calculated between all planet pairs, with configurable orbs per aspect type. Secondary progressions, solar arc directions, solar returns, and transits all use the same DE441 base.

Precision and validation

We validate our engine against NASA's JPL Horizons system — the same tool used by professional astronomers. Validation is ongoing and automated; the figures below update as new batches complete.

Solar positions are accurate to 0.12'' RMS. Overall planetary accuracy is approximately 1.2'' (excluding Pluto, which has higher variance due to ecliptic reference frame differences). Any divergence greater than 1' (one arcminute) triggers a diagnostic review.

During this validation, we identified cases where widely-used astrology platforms assign a planet to the wrong zodiac sign near sign boundaries — positions where our engine matches NASA exactly. We document this not as criticism, but because it illustrates why the choice of ephemeris matters in practice, not only in theory.

Our target is 100,000 charts verified against NASA Horizons by the end of 2026.

Modern rulerships

Astrian uses modern planetary rulerships: Pluto rules Scorpio, Uranus rules Aquarius, Neptune rules Pisces. This is a deliberate editorial choice. Traditional rulerships — Mars for Scorpio, Saturn for Aquarius, Jupiter for Pisces — are documented in our glossary for reference and context.

What we don't claim

Astrian presents astronomical data and the interpretive tradition built around it. We don't predict the future. We don't claim that planetary positions cause events. A chart is a map — what you do with that map is yours.

Open about our limits

House systems are a human convention: the sky doesn't divide into twelve equal sectors. Which system fits your practice is an open question we won't answer for you. The house system comparator exists precisely because there is no single correct answer.

Birth time matters enormously for the Ascendant and house cusps. A difference of four minutes shifts the Ascendant by roughly one degree. If your birth time is unknown or uncertain, your house cusps and rising sign may be unreliable. The chart positions for the Sun, Moon, and planets will still be accurate for any known birth date.

No astrological software can tell you who you are. A chart is a starting point for inquiry, not a conclusion.